Loudspeaker diaphragms



Dec. 31, 1957 A, H. wHlTELEY 2,818,130

LOUDSPEAKER DIAPHRGMS Filed April 15, 1954 2 sheets-sheet 1 l a), KUL-A TTOHVEY Dec. 31, 1957 A. H. WHITE-:LEY 2,818,130

LOUDSPEAKER DIAPHRAGMS Filed April 13, 1954 2 Sheets-Sheet 2 INVENTOR /L raso /noLo //wns-ir A TTORNEY United States liatent LOUDSPEAKER DIAPHRAGMS Alfred Harold Whiteley, Mansfield, Nottingham, England, assignor to Whiteley Electrical Radio Company Limited, Mansfield, Nottingham, England, a British company Application April 13, 1954, Serial No. 422,866 Claims priority, application Great Britain April 13, 1953 l2 Claims. (Cl. 181--32) This invention relates to loudspeakers, microphones and like electroacoustic transducing devices employing diaphragms, to diaphragms for use therein, and to methods of manufacturing such diaphragms.

In accordance with the invention there is provided a method of manufacturing a diaphragm for an electroacoustic device, which comprises applying to a fibrous sheet material a hardenable synthetic resin, forming said sheet material and at least partly hardening said resin, applying to the formed sheet material a further iibrous material, and bonding said materials together.

Other features and advantages of the invention will appear from the following description or" one method of manufacturing a diaphragm, in accordance with the invention, and of a loudspeaker incorporating a diaphragm so. made, in conjunction with the accompanying drawings in which:

Figures 1, 2, 3 and 4 are half-sections of a diaphragm in theA process of manufacture, and

Figure 5 is a side elevation, half in section, of the complete loudspeaker.

The diaphragm is made from a sheet of loosely woven linen or cotton cambric, rather larger than the ultimate diameter of the diaphragm and its surround. Being woven, this material is, of course, coherent and selfsupporting, but it is of open Weave having, in this particular example, about 50 threads to the linear inch. This sheet material is lightly impregnated with a hardenable synthetic resin, such as an incompletely cured resin. We have used with success for this purpose a material sold by Bakelite Ltd. as Bakelite FBl1233, but other resins can be used.

This material is lirst formed to the shape of the diaphragm which` is required. ln this example, the diaphragm is of the type intended for use in a loudspeaker of the moving coil type, and accordingly being of generally conical, truste-conical or elliptical shape. The cambric is therefore placed in a heated shaping press; with the resin described a heating temperature in the range 30G-500 F. is suitable, with a heating time of l() to l5 seconds, When the cam-bric is removed from the press the resin upon it has been cured and hardened to the point where the cambric has a substantial degree of permanent stiffness, and has a shape as shown in Figure l, with a frusto-conical part 1i) and a marginal surround 11 including corrugations 12. The cambric is also formed with a shoulder at 13, and a central disc 14.

The stiilened cambric next has a layer of a fibrous material deposited in a felted condition upon it. The fibres which can be used for this purpose include those of cotton, sisal, wood pulp or cellulose; the fibres are broken down to a small size by a suitable heating mechanism. A suspension of the bres in Water is made, and is maintained by agitation, and the deposit is prepared by passing the lSuspension, by suction or pressure, through the permeable cambric. This process takes only a short time, and a suilicient deposit can be obtained in a matter of a few seconds.

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The nbre is deposited over the conical area of the diaphragm, as shown diagrammatically at 1S in Figure 2, the surround being left free, and in the example of Figure 2 the deposit is shown as being of uniform thick This is not necessarily the case, and depending upon the results desired the deposit can be varied in thickr s locally, or the variation can be gradually varied over ea of the diaphragm. Thus, a heavier deposit can be made at the base or apex of the conical part and it can be varied round the conical part, so as to break up resonances which may tend to occur. Additionally different materials can be used at different parts, and deposits can be made on both sides of the cambric. It is also possible for the thickness and/or weight of impregnation of the cambric to be varied for the same reason, and also apertures can be cut in the cambric if desired.

in a practical case, in a diaphragm having an effective diameter' of about 71/2 inches, it has been found suitable to produce a substantially uniform deposit, made of wood. libres, weighing in the dry state approximately 3 grams which is about the weight of the area of the impregnated cambric that is effective as a diaphragm.

When the libres have been deposited on the formed cambric they are loosely felted and the deposit is relatively thick, as is indicated in Figure 2. The cambric with the wet deposit is then placed in a press, rather similar to that lirst used but having at least one face covered with a metal gauze. On application of heat and pressure the wet deposit is dried and compacted, the gauze permitting the escape of steam. The resulting layer is then of the same order of thickness as the cambric, as indicated in Figure 3.

rPhe cambric with its deposit is allowed to dry cornpletely and the deposited area is then coated with a material such as a cement indicated at 16 in Figure 4, the purpose of which is to bond the fibres of the deposit to those of the cambric. The cement can be of the solvent type, which improves penetration and/or it can be of a type including a thermoplastic or thermosetting material, and it may be compatible with the resin on the cambric. A suitable material is a cellulose stoving enamel, made by Cellon Ltd. and sold by them as Cerrux low bake stoving enamel, clear, 2SH-094. A coating oi' this material is applied to the exposed face of the cambric and after the solvent has been allowed to evaporate the composite diaphragm is again placed in a heated press.

This last heating under pressure causes the enamel both to achieve better penetration and also to harden, so that the necessary bonding is obtained and in addition the stiffness of the diaphragm is increased, and the high frequency response improved.

As any alternative to the use of a separately applied coating of cement to bond together the cambric and the fibres, a further material can be included in the liquid used for the suspension, `so that compacting drying and bonding of the fibres can be achieved in one heating and pressing operation. lt is, however, difficult to iind a completely satisfactory material for this purpose, and it is therefore preferred to apply the cement separately.

The surround of the diaphragm is sutliciently permeable to allow sound radiation to pass from the front to the back of the diaphragm in use, which is not usually desirable. To prevent this the surround can be given a coating of material such as rubber solution which seals the interstices in the cambric, but which does not greatly reduce its compliance.

lt will be appreciated that in Figures l to 4 the thick nesses of the parts have been exaggerated for clarity, and that the heat and pressure to which the parts are subjected also causes the layers to penetrate one into an` Ii other; in particular the layer 16 will penetrate the other layers.

Figure shows, by way of example, a loudspeaker using a diapragm such as that described. The disc 14 is punched out of the diaphragm and a coil support 17, with coil 18 is attached to it. A centering spider 19 is cemented to the assembly, which is then mounted in a housing 20 with the coil in the air gap of a magnetic structure 21. The centering spider 19 can with advantage be made of the same material, and by the same method of manufacture as that employed for the diaphragm. The diaphragm is secured in position with the margin of the surround secured to the housing, with a felt or like clamping ring 22 over it. The coil is connected to terminal tags 24, and a dust cover 25 cemented in position.

It is Well known that for a loudspeaker to have an extended frequency response it is necessary for the moving system to have a bass resonant frequency, which is as low as possible. With the present invention it has been found possible to reduce the resonant frequency of a loudspeaker to almost half the ligure obtainable with a conventionally made diaphragm. This improvement is made possible Without recourse to elaborate suspensions sometimes adopted, and in addition a diaphragm made in accordance With the invention is more stable over a period of time.

I claim:

l. A method of manufacturing a diaphragm for an electroacoustic device comprising treating a sheet of open woven fabric by applying a hardenable binder thereto, at least partly hardening said applied binder, applying to the treated sheet a deposit of fibrous material, and bonding the deposited fibrous material to the sheet.

2. A method of manufacturing a diaphragm for an electroacoustic device comprising applying to a sheet of open woven fabric a hardenable binder to an extent sufficient to both impregnate the material of the fabric and maintain the permeability of the sheet, at least partly hardening said binder, applying to the thus treated sheet a deposit of fibrous material, and bonding the deposited fibrous material to the sheet.

3. A method of manufacturing a diaphragm for an electroacoustic device comprising applying to a sheet of open woven fabric a hardenable binder, forming said sheet to a desired configuration and simultaneously at least partly hardening said binder, and applying a deposit of fibrous material to the sheet and bonding the fibrous material and the sheet together.

4. A method of manufacturing a diaphragm for an electroacoustic device comprising applying to a sheet of open woven fabric a hardenable binder, at least partly hardening said binder, applying a deposit of fibrous material Within a peripheral margin defined by the sheet, and simultaneously compacting said fibrous material on the sheet and bonding said fibrous material to the sheet.

5. A method of manufacturing a diaphragm for an electroacoustic device comprising applying to a sheet of open woven fabric a hardenable synthetic resin, simultaneh ously at least partly hardening said resin and forming said sheet to a desired configuration, applying a deposit of fibrous material within a peripheral margin defined by the formed sheet, compacting said fibrous material on to said sheet, and bonding said fibrous material to said sheet.

6. A diaphragm for an electroacoustic device comprising a layer of open Woven fabric impregnated with a hardened binder and a layer of fibrous material bonded thereto.

7. A diaphragm for an electroacoustic device comprising a permeable layer of open Woven fabric impregnated with a hardened binder and a felted fibrous mass bonded to said layer.

8. A diaphragm for an electroacoustic device comprising a permeable layer of open woven fabric impregnated with a hardened synthetic resin, and a felted fibrous mass compacted upon and bonded to said layer.

9. A diaphragm for an electroacoustic device comprising a permeable layer of open Woven fabric impregnated with a hardened synthetic resin, said impregnated fabric being of predetermined shape having a conical region and a marginal border, and a felted fibrous mass compacted upon and bonded to the conical region of said layer.

10. A diaphragm for an electroacoustic device com.- prising a permeable layer of open Woven fabric impregnated with a hardened synthetic resin, said impregnatedy fabric being of predetermined shape having a frustoconical region and a marginal border, and a felted fibrous mass compacted upon and bonded to said frusto-conical region, and a coating of flexible material rendering im.- permeable said marginal border, the compacted mass of. fibrous material having a different Weight per unit area at predetermined parts of the diaphragm.

ll. A diaphragrm as claimed in claim l0 in which the total weight of compacted material is substantially equal to the weight of that part of the impregnated layer to which it is bonded.

12. An electroacoustic device comprising a diaphragm defined by a layer of open woven fabric impregnated with a hardened binder and a felted fibrous mass compacted upon and bonded to said layer, and a centering device for said diaphragm, said centering device being composed of a material which is the same as that of said diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS 2,006,830 Hawley July 2, 1935 2,017,496 Hawley Oct. 15, 1935 2,087,441 Metcalf et al July 20, 1937 2,146,975 Nagelvoort Feb. 14, 1939 2,152,901 Manning Apr. 4, 1939 2,206,517 Steffens July 2, 1940 2,502,853 Keddie Apr. 4, 1950 2,549,091 Hopkins Apr. 17, 1951 2,596,364 Brennan May 13, 1952 y 2,624,417 Brennan Ian. 6, 1953 

